Light emitting diodes (LEDs) are currently used as replacements for incandescent light bulbs and fluorescent lamps. LEDs are semiconductor devices that emit incoherent narrow-spectrum light when electrically biased in the forward direction of their PN junctions, and are thus referred to as solid-state lighting devices.
Solid-state lighting devices are well established in applications such as street signs, traffic and railroad signals, and indicator lamps for trucks and automobiles. High output LED fixtures suitable for general architectural lighting applications are beginning to appear on the market with system efficiencies comparable to fluorescent systems.
Performance of an LED largely depends on the ambient temperature of the operating environment. Driving an LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Therefore, adequate thermal management is required to maintain a long operating life. This is especially important when considering applications where the device needs to operate over a large range of temperatures, and is required to have a low failure rate.
One approach for dissipating heat for LEDs mounted on a printed circuit board is to use a printed circuit board comprising a metal core as compared to traditional printed circuit boards comprising a dielectric core. While metal core printed circuit boards are effective for dissipating heat, disadvantages are increased costs and processing difficulties. In addition, since there are limitations to the size of metal core printed circuit boards, they are more difficult to incorporate into larger size devices.
Another approach for dissipating heat from LEDs is to attach the LEDs directly to a heat sink using a thermally conductive adhesive or tape. A disadvantage of this approach is that it is a labor-intensive process, resulting in increased costs. In addition, the resulting configuration is subject to high failure rates.
Yet another approach is disclosed in U.S. Pat. No. 6,045,240 in which LEDs are on a front side of the printed circuit board, and a heat sink is on the back side of the board. To increase the transfer of heat from the circuit board to the heat sink, the printed circuit board has a plurality of holes extending there through, and a plurality of thermally conductive pads is within the printed circuit board. The pads are of a thermally conductive plating, and each pad is associated with at least one of the holes for conducting heat from each of the leads of the LEDs to one of the pads.
A further approach is disclosed in U.S. Patent Application Publication No. 2007/0081340 in which an LED is mounted over one or more holes. The holes are lined and/or filled with a thermally conductive material to dissipate heat from the LED. The holes extend through the printed circuit board. As a result, the thermally conductive material in each hole is exposed during the manufacturing process. A disadvantage of this approach is that the exposed thermally conductive materials may have a low melting point. As a result, the exposed thermally conductive material may fall out during subsequent soldering steps involving temperatures greater than the melting point, thus rendering the filled holes useless.
Even in view of the different approaches for dissipating heat generated by LEDs mounted on a printed circuit board, there is still a need to improve thermal management thereof as well as forming the heat dissipating LED assemblies.